Doubts and questions regarding breeding carriers have been present among dog breeders since first development of modern dog breeds. With the fast development in the field of genetics and molecular biology, a number of DNA tests became available for purebred dogs. These tests became a powerful tool in the control of the quality of one’s dog, but DNA tests also enabled planning of breeding program on a whole new level. However, it has been noticed that breeders often seem shocked or in a way disappointed when their dog tests as a carrier for specific mutation.In the following article we would like to discuss what does ”carrier” in the test certificate exactly mean, whether carriers should be included in the breeding program, as well as break the stigma regarding this genetic status.
Closed gene pool in purebred dogs
Since prehistoric time and first wolf domestication, humans are selecting which dogs to breed based on desired traits and characteristics. Through this practice, today we have developed specific dog breed groups: hunting, shepherding, guarding, work and company breeds. More about dog breeding history you can read here.
Dogs with desirable traits will be used more often in the breeding program, while dogs lacking those traits will not be used at all. This means that the popular sire or dam will transfer its genetic material more often to offspring, while the genetic material of dogs excluded from breeding will not be passed on on the future generations.
Such selective breeding led to extreme shrinkage of the pure breed dog’s gene pool. Smaller gene pool increases the risk of inbreeding or mating dogs that are closer or distant relatives, which in turn also increases the risk of appearance of inherited disorders among dogs. The reduced gene pool is exactly the reason why pure breed dogs are often more prone to diseases, compared to generally healthy mixed-breed dogs.
Gene pool is a total number of all genes in a certain population.
Most of the genetic disorders are inherited in an autosomal recessive pattern. Each dog has two specific genes, each inherited from one parent. Genes for a specific disorder can be normal (do not cause the disorder) or can be mutated (can cause the disorder in case dog has two copies of the mutated gene).
An example of disorder with simple inheritance are many inherited eye disorders, such as progressive retinal atrophy (PRA). Before the DNA tests were developed, dog’s eyes were examined by the veterinarian. However, such examinations do not provide fully reliable information, since symptoms of inherited disorders often do not develop until later in life, after the dog has already entered the reproductive age. This means that affected dog will mate and pass on the genes that cause disorder, without the owner even knowing that the dog is affected.
Keeping the gene pool stable – breeding carriers
The only way to be sure of your dog’s genetic heritage is DNA testing. Responsible breeders are aware of the importance of such tests and get their dogs tested in order to maintain or improve the quality of their dogs. On the other side, often when the results show that their dog is a carrier for certain autosomal recessive disorder, owners appear confused or disappointed. Considering centuries of inbreeding among pure breed dogs, the appearance of carriers is not unusual. Additionally, some breeders appear discouraged from using their carrier dogs in the breeding program. Although the dog carries one copy of the causative mutation, other copy of the allele is healthy, and the carrier dogs will be completely healthy dogs considering a specific disorder. When breeding carrier with a clear dog, there is 50% chance that puppies will be clear and 50% chance that puppies will be carriers, but ALL PUPPIES WILL BE HEALTHY. On the other side, excluding carrier completely from the breeding program will reduce the gene pool for this dog’s genetic material. Carrying out this practice throughout years would cause dramatic shrinkage in the gene pool, which in turn, can lead to appearing of some new, until now unknown or extremely rare disorders.
Autosomal recessive inheritance patterns
An extreme gene pool shrinkage can be on the example of the Norwegian Puffin Dog. At some point in the 20th century, there were only 5 dogs of this breed left. Today the breed is well preserved, but a breed specific disorder is occurring in this breed, the Lundehund syndrome, with an estimated prevalence of the disorder up to 100%. While today carrier rate of 10% is already considered high, this is an example of extreme gene pool shrinkage.
Like many other purebred dogs, the Lundehund went through severe gene pool shrinkage.
New era, new approach
Over the years it was noticed that excluding dogs from the breeding program was not sufficient for eliminating disorders from the breed and phenotypical examination of the dog’s pedigree and family tree provided good but not complete information about the quality of the future litter. Today’s development in the molecular diagnostics allows a completely new approach to dog breeding based on the genetic heritage, unlike diagnostics based on already developed symptoms or visible phenotype. Recognition of carrier dogs allows their inclusion in the breeding program, with obtaining healthy litters when mating with clear dogs, and at the same time it enables avoiding mating two carriers or affected dogs and appearance of affected puppies. Simultaneously, keeping carriers in cleverly planned breeding program will maintain the stability of the genetic pool, without increasing the risk of new, unknown disorders emerging. Genetic tests for dogs offer an easy and simple way for identification of genetic heritage of your dog and improved quality of future litters, without deleterious effects of gene pool shrinkage.
Marsden CD, Ortega-Del Vecchyo D, O’Brien DP, et al. Bottlenecks and selective sweeps during domestication have increased deleterious genetic variation in dogs. Proceedings of the National Academy of Sciences of the United States of America. 2016;113(1):152-157. doi:10.1073/pnas.1512501113.